Conventional network-based communication systems include systems configured to operate in accordance with well-known synchronous transport standards, such as the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) standards.
The SONET standard was developed by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), and is described in the document ANSI T1.105-1988, entitled “American National Standard for Telecommunications-Digital Hierarchy Optical Interface Rates and Formats Specification” (September 1988), which is incorporated by reference herein. SDH is a corresponding standard developed by the International Telecommunication Union (ITU), set forth in ITU standards documents G.707 and G.708, which are incorporated by reference herein.
The basic unit of transmission in the SONET standard is referred to as a synchronous transport signal level-1 (STS-1). It has a serial transmission rate of 51.84 Megabits per second (Mbps).
Synchronous transport signals at higher levels may be concatenated or channelized. For example, an intermediate unit of transmission in the SONET standard is referred to as synchronous transport signal level-3, concatenated (STS-3c). It has a serial transmission rate of 155.52 Mbps. The corresponding unit in the SDH standard is referred to as STM-1. In a concatenated synchronous transport signal, the entire payload is available as a single channel. A channelized signal, by way of contrast, is divided into multiple channels each having a fixed rate. For example, the channelized counterpart to the concatenated STS-3c signal is denoted STS-3. STS-3 is a channelized signal that comprises three separate STS-1 signals each at 51.84 Mbps.
A given STS-3c or STM-1 signal is organized in frames having a duration of 125 microseconds, each of which may be viewed as comprising nine rows by 270 columns of bytes, for a total frame capacity of 2,430 bytes per frame. The first nine bytes of each row comprise transport overhead (TOH), while the remaining 261 bytes of each row are referred to as a synchronous payload envelope (SPE). Synchronous transport via SONET or SDH generally involves a hierarchical arrangement in which an end-to-end path may comprise multiple lines with each line comprising multiple sections. The TOH includes section overhead (SOH), pointer information, and line overhead (LOH). The SPE includes path overhead (POH). Additional details regarding signal and frame formats can be found in the above-cited documents.
In conventional SONET or SDH network-based communication systems, synchronous transport signals like STS-3c or STM-1 are mapped to or from corresponding higher-rate optical signals such as a SONET OC-12 signal or an SDH STM-4 signal. An OC-12 optical signal carries four STS-3c signals, and thus has a rate of 622.08 Mbps. The SDH counterpart to the OC-12 signal is the STM-4 signal, which carries four STM-1 signals, and thus also has a rate of 622.08 Mbps. The mapping of these and other synchronous transport signals to or from higher-rate optical signals generally occurs in a physical layer device commonly referred to as a mapper, which may be used to implement an add-drop multiplexer (ADM) or other node of a SONET or SDH communication system.
Such a mapper typically interacts with a link layer processor. A link layer processor is one example of what is more generally referred to herein as a link layer device, where the term “link layer” generally denotes a switching function layer. Another example of a link layer device is a field programmable gate array (FPGA). These and other link layer devices can be used to implement processing associated with various packet-based protocols, such as Internet Protocol (IP) and Asynchronous Transfer Mode (ATM), as well as other protocols, such as Fiber Distributed Data Interface (FDDI). A given mapper or link layer device is often implemented in the form of an integrated circuit.
In SONET/SDH mapper applications, overhead information such as the above-noted TOH information may be used for network control, status reports, and other functions. It is typical for only a single byte of such information to be used for a given instance of such a function. One reason for this is that it may be difficult under certain circumstances to ensure that multiple consecutive bytes of the overhead information are inserted in a consistent manner within a given frame. For example, if the TOH bytes are inserted under the control of a host processor that operates with a different clock frequency or phase than that of the mapper, a consistent sequencing of multiple TOH bytes in the given frame is not guaranteed. That is, multiple TOH bytes that would need to be in a particular order so as to provide a desired function are not guaranteed to be inserted in that particular order within the given frame.
Accordingly, a need exists for an improved approach to insertion of TOH bytes or other overhead information in forming a synchronous transport signal or other type of signal.